The answer is: we should not and we can not.And yet again (I say this to the regular CC guys answering this thread - you know who we are...) we have the customer supplying the appliance, and querying the technical knowledge of his installer..... should we as GasSafe installers presumably with a living to make, be answering any such queries?
It may be true that the calculation method can be looked up in a book, but it is not that easy to apply. It is possibly the number on issue training gas opertives struggle with, and it would not be wise attempting to diy the calculation in order to check whether the installer got it right.
On top of that, without being on site we can not tell if a chosen route is acceptable.